We recently showed that offspring delivered to estrogen (E2)-suppressed baboons exhibited insulin resistance, glucose intolerance, and a deficit in first phase insulin secretion, steps that progress to type 2 diabetes mellitus (T2DM). However, the mechanism(s) underpinning this E2 regulated event are unknown. The microvessel (MV) unit (i.e. arterioles and associated capillaries [cap]) has a fundamentally important role in insulin action by enabling insulin and glucose delivery to target tissue, notably skeletal muscle (SM). An extensive MV network forms within insulin target tissues during fetal development, however, little is known about the regulation of this critically important developmental process in the fetus. Angiogenesis is foundational for expansion of the cap network during fetal development and vascular endothelial growth factor-A (VEGF) is a predominant regulator of angiogenesis in SM. Therefore, the over-arching highly novel concept of this ?developmental origin of health and disease? study is that E2 in utero promotes SM MV development in the fetus and consequently formation of an extensive MV network critical for the delivery of insulin and glucose to and thus insulin action and glucose homeostasis within SM in the offspring. In Aim 1, we will test the hypothesis that E2 promotes SM MV morphological and functional development in the baboon fetus as an essential step leading to insulin sensitivity in the offspring. SM VEGF expression, cap density and MV maturation and morphology will be quantified in the fetus at mid (day 100) and late (days 165-175; term = 184 days) gestation and in offspring at 2, 3 and 4 years of age delivered to baboons untreated or in which E2 production/levels have been suppressed by maternal administration of the aromatase inhibitor letrozole and restored by letrozole plus E2. SM vascular function will be assessed by brachial artery flow-mediated dilation and by cap flow, as quantified by contrast-enhanced ultrasound/microbubble technology, before/during vasochallenge of offspring delivered to E2-deprived/-replenished baboons. Aim 2 will determine the mechanisms by which E2 acts to promote SM angiogenesis in the fetus as established in Aim 1. We will test the hypothesis that E2 rapidly stimulates SM VEGF expression, cap endothelial cell (EC) tight junction (TJ)/adherens junction (AJ) breakdown, and cap EC proliferation as early steps in angiogenesis on day 165 of gestation 0-24 h after an iv bolus injection of E2 to fetuses of letrozole-treated baboons. The proposed study is clinically significant since preterm birth, aromatase mutation, steroid sulfatase deficiency, estrogen receptor null mutation, and maternal/fetal exposure to endocrine disruptors, which curtail exposure of the fetus to the normal elevation in or action of E2, are associated with increased incidence of insulin resistance/T2DM in human offspring. Establishing the importance of E2 to fetal MV development and onset of insulin sensitivity in primate offspring provides a basis for therapeutic application to the human.